This invention generally relates to a permanent magnet having magnetic anisotropy by means of mechanical orientation, and a manufacturing method thereof. More particularly, the present invention relates to a permanent magnet comprising R (R being at least one element selected from the group consisting of rare earth elements including Yttrium (Y)), M (M being at least one element selected from the group consisting of transition elements such as Fe, Cu, Au and Zr) and X (X being at least one element selected from the group consisting of B, Al and Ga).
Permanent magnets are important electric-electronic materials which are used in a wide range of fields such as in domestic appliances and in peripheral equipment for computers. In pursuit of the present users' desire for miniaturization and the improved of efficiency of permanent magnets, higher performance permanent magnets have been required.
Permanent magnets are made of a material which can produce magnetic fields without applying electric power, has a high coercive force, and has a high residual magnetic flux density. These requirements are quite different from high permeability magnetic material, which is currently used in magnets of the Alnico series, barium-ferrite and the rare-earth transition metal series.
In particular, permanent magnets of the rare-earth transition metal series, such as R-Co or R-Fe-B magnets, have high magnetic properties having very high coercive forces, and energy-product values, therefore, much research and development has been carried out on them.
Described below are several references concerning high performance anisotropic permanent magnets of the rare-earth-iron (transition metal) series and their manufacturing method:
(1) Japanese patent application disclosure No. 59-46008 (equivalent to EP 101552A and U.S. Pat. No. 4,770,723), and in the reference by M. Sagawa, S. Fujimura, N. Togawa, H. Yamamoto and Y. Matsuura (J. Appl. Phys. Vol. 55(6), 15 Mar. 1984, p. 2083), disclose permanent magnets which are characterized by a magnetically anisotropic sintered substance comprising 8-30 atomic % of R (R being at least one element selected from the group consisting of rare-earth elements including Y) and residual of iron(Fe). This substance is manufactured by means of a sintering method of powder metallurgy.
In this sintering method, the manufacturing process comprises preparing an alloy ingot by means of melting and casting, providing magnetic powder of suitable grain size by means of grinding, blending said powder with an additive binder for forming and forming a green body by press-forming in a magnetic field. After pressing, the green body is sintered in an argon atmosphere at a temperature of 1100 degree centigrade for about one hour, and after that, the product is rapidly cooled to room temperature. After sintering, the product is heat-treated at 600 degree centigrade to improve its coercive force.
(2) Japanese patent application disclosure No. 59-211549 (equivalent to EP125752), and the reference by R. W. Lee (Appl. Phys. Lett. vol. 46(8), 15 Apr. 1985, p790) disclose a resin-bonded rare-earth-iron magnet which is formed from fine particles of alloy ribbon prepared by means of the melt-spinning method, having a fine crystalline magnetic phase and comprising at least one rare earth element selected from the group consisting of neodymium, praseodymium and mesh metal, transition metal elements, and iron and boron. The fine particles are formed in the desired shape of a magnet by a binder mixed with the particles, the fine particle being magnetically isotropic and the formed magnet being magnetized to any desired direction in a proper magnetic field. The magnet has a density of at least 80% of the alloy density and an energy product of at least 9 megagauss-oersted.
The permanent magnet is manufactured by means of the resin-bonding method using rapidly quenched thin ribbon prepared by the melt spinning method having a process comprising rapidly quenching thin ribbon of about 30 micrometer thickness by means of a melt spinning apparatus used to provide an amorphous alloy.
In the resin bonding method, using a rapidly-quenched ribbon prepared by the melt-spinning method, a first rapidly-quenched thin ribbon of R-Fe-B alloy is prepared by means of a melt spinning apparatus. The obtained ribbon of 30 micrometer thickness is an aggregate of crystal having a diameter of less than 1000 micrometers, and is brittle, and easily breakable, and the crystals are distributed homogeneously so that it has an isotropic magnetic property. It is able to obtain a magnet of density of more than 85% by means of press forming pulverized particles obtained by pulverizing the thin ribbon to a desired grain size with a resin under a pressure of about 7 tons/cm.sup.2.
(3) Japanese patent application disclosure No. 60-100402 (equivalent to EP 133758A), and in the reference of R. W. Lee; (Appl. Phys. Letter. Vol. 46(8), 15 Apr. 1985, p790.). These references describe:
(a) Isotropic permanent magnets comprised of fully densified fine particles characterized in that the magnet being provided by hot pressing amorphous or fine particle material comprises iron, neodimium and/or praseodimium and boron.
(b) Anisotropic permanent magnets consisting of fine particles characterized by providing a magnet by hot pressing and hot die upsetting a material comprising iron, neodimium and/or praseodymium and boron, the desirable magnetizing direction of the magnet being parallel to the upset compression direction.
(c) Permanent magnets characterized by a magnet formed by high temperature plastic deforming of an amorphous or fine particle alloy comprising substantially 50.about.90 atomic % of iron, 10.about.50 atomic % of neodymium and/or praseodymium and 1.about.10 atomic % of boron in which the desirable magnetizing direction is perpendicular to the plastic flow in the deformation.
These references also disclose a method of manufacturing a permanent magnet, wherein the permanent magnet is anisotropic and characterized by being composed of iron-rare-earth metals. The manufacturing method comprising heat treating amorphous or fine grained solid particles, including iron, neodymium and/or praseodymium and boron, to prepare a plastically deformed body of fine grained microstructure, and cooling the body.
The manufacturing method of these magnets is a method to manufacture R-Fe-B magnet having anisotropic properties and having a high density by means of a 2-step hot-pressing method in a vacuum or in an inert-gas atmosphere from a ribbon-like rapidly-quenched thin ribbon or plate.
In this pressing process, one-axial pressure is applied to align the magnetization direction parallel to the pressing axis to prepare an anisotropically magnetizable alloy.
Also, it is preferable that the particle grain size of the ribbon-like thin plate be preliminarily manufactured by the melt-spinning method and may be prepared smaller than the grain size showing maximum coercive force to give optimum grain size after the grain-growth in the hot-press process.
(4) Finally, Japanese patent application disclosure No. 62-276803 (equivalent to DE3626406A or U.S. patent application No. 06/895,653) discloses a permanent magnet of a rare-earth-iron system which is characterized by melting an alloy comprising 8.about.30 atomic % of R (at least one element selected from the group consisting of rare-earth elements including Y), 2.about.28 atomic % of boron, less than 50 atomic % of cobalt, less than 15 atomic % of aluminum and rest iron and impurities. The alloy is casted and hot-worked at a temperature above 500.degree. C. so as to refine the crystal grain and orient crystal axis to a specific direction to make a magnetic anisotropic cast alloy.
The permanent magnet of R-Fe-B system described above in sections (1) to (4) have drawbacks described below.
The manufacturing method described in the references of section (1), it is indispensable to pulverize the alloy, and because of that, the R-Fe-B alloy is very active to oxygen, and when it is pulverized, the power becomes highly oxidized.
Also, in the process of the forming the powder, it is necessary to add forming additive such as zinc stearate, although this additive may be removed preliminarily before the sintering process, some part of the additive remains in the magnet body in the form of carbon, and it is not desirable because this residual carbon deteriorates the magnetic properties of the R-Fe-B magnet.
The formed body, which is called a green body, is very difficult to handle because it is easy to break. Thus, the handling is very troublesome when the formed bodies are arranged in the sintering furnace.
As a result of these drawbacks, the manufacturing cost of a magnet of a R-Fe-B series, is expensive because it is not only necessary to provide expensive equipment, but in addition the manufacturing method thereof does not have good productivity. Thus, it is not able to effectively utilize of an advantage of a R-Fe-B series magnet, i.e., cheap raw materials.
The permanent magnets described in sections (2) and (3) are manufactured by means of vacuum-melt spinning machine. This machine is not only very expensive, but also has very low productivity.
The permanent magnet describe in section (2) is also disadvantaged in that the temperature characteristic and the application thereof because it has homogeneous magnetic properties so that it is a low energy product and also has bad squareness of the hysteresis loop.
The manufacturing method described in section (3) is a unique one which utilizes hot-pressing in two-steps, but when used for mass-production, it is inefficient.
Furthermore, coasening of the crystal grains is remarkable if the temperature rises above 800.degree. C. Because of which, the intrinsic coercive force iHc becomes extremely low so that it is not able to provide a practical permanent magnet.
With respect to the manufacturing method described in section (4), it has the drawback that the manufactured magnet has somewhat inferior magnetic properties compared with that of the magnets manufactured in accordance with references described in sections (2) or (3), although it does not include a pulverizing process and has only one hot-press process thus reducing the manufacturing process to its maximum extent.